Polymer electrolyte fuel cells (PEFCs) are attractive for consumer applications because of their relatively low operating temperature and/or non-corrosive electrolyte compared to other types of fuel cells. On the other hand, the oxygen reduction reaction (ORR) in the strongly acidic environment is not particularly facile and the water management in the PEFC is complicated by the competing needs of adequate hydration of the electrolyte membrane and minimal "flooding" of the gas access channels to the membrane surface. The usual strategy for maximizing the ORR rate in terrestrial applications (i.e., air operation) consists of pressurizing the air for the cathode to maximize the oxygen partial pressure and facilitate its transport in the presence of the nitrogen inert component.
Water management typically consists of humidifying the reactant gases and using hydrophobic materials where appropriate to minimize the accumulation of liquid water. The polymer electrolyte membrane is then hydrated by contact with the water vapor in the reactant streams and by the ORR product water. Optimal hydration is difficult to attain over the full operating envelope with this approach because the membrane tends to dry out at low current densities and the cathode structure tends to "flood" at high current densities where removal of product water becomes an issue.
To balance the needs to pressurize and/or humidify the reactant gases for good performance, a number of auxiliary subsystems become vital. In most cases, it is also necessary to provide a means to cool the fuel cell stack. While this can be integrated into the humidification subsystem in some manner, it typically requires that some manner of cooling plates or cells be integrated into the fuel cell stack, resulting in greater weight, size and complexity. Often, the performance of individual cells within the stack depends upon their location relative to a cooling plate.
One of the major difficulties with such complex systems is the parasitic power required to pressurize the air for the fuel cell stack. An expander on the downstream side can be used to recover some of the power expended. But a state-of-the-art system, such as that developed by Ballard to operate at 3 atm (30 psig), still loses about 20% of its gross power to auxiliary systems, with most of the power loss arising from the compressor. In addition, the compressor/expander system is also relatively large, complicated and expensive. The use of compression also limits the amount of excess air that can be introduced into the cathode plenum. While a substantial excess of air can considerably improve performance, the power requirement for compression quickly overwhelms the advantages gained and typical flows utilized for the cathode air are on the order of two times the stoichiometric flow (or about 50% oxygen utilization).
These considerations illustrate some of the difficulties and challenges inherent in polymer electrolyte fuel cell systems. A number of these difficulties can be alleviated with the use of an effective means for introducing liquid water directly to the membrane/electrode assembly (MEA) instead of humidifying it indirectly via the reactant gases. The advantages of direct liquid hydration have been described by Watanabe et al. (140 J. Electrochem. Soc., pp. 3190 (1993)). Watanabe and Cisar et al. (U.S. Pat. No. 5,635,039), both incorporated herein by reference, have developed internal membrane structures for delivering liquid water directly to the ionomeric membrane. U.S. patent application Ser. No. 08/810,229, filed Feb. 24, 1997, by M. S. Wilson now U.S. Pat. No. 5,592,119 issued Sep. 14, 1999, and incorporated herein by reference, describes another approach that uses "mixed" hydrophobic/hydrophilic gas diffusion backings adjacent the MEA to convey liquid water from separate channels in the anode flow-field directly to the MEA. Liquid water is then introduced to flow-field channels through manifolds and distribution channels similar to the hydrogen distribution of many stack designs. With the MEAs in direct liquid contact with a water reservoir, the membranes stay nearly fully hydrated even at elevated temperatures or low current densities without the need for reactant humidification.
The present invention is directed to a system having fully hydrated membranes that overcomes the problems inherent in pressurized fuel cells using humidified reactant gases. Accordingly, it is an object of the present invention to operate the air cathodes of a fuel cell system at near ambient pressure.
Another object of the present invention is to provide a non-humidified or dry air stream to the fuel cell cathode.
One other object of the present invention is to provide a sufficiently high flow-rate of cathode gas to remove water formed at the cathode so there is no significant accumulation of water.
Still another object of the present invention is to cool the fuel cell stack by direct evaporation of water from the cathode side of fuel cell membrane-electrode assembly (MEA) into the reactant gas in the cathode flow field.
Yet another object of the present invention is to minimize parasitic power losses while providing reactant gases to the fuel cell.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.